The present invention relates to a thermal type air flow meter, and particularly to an intake air flow meter for measuring an air flow rate taken into an internal combustion engine.
An analog-to-digital (A/D) conversion method of detecting an air flow rate taken into an internal combustion engine and then performing analog-to-digital conversion is disclosed, for example, in Japanese Patent Application Laid-Open No. 2-85724. This apparatus comprises an air flow rate detecting unit having a bridge circuit and detection circuit for detecting an air flow rate, and an electrical control unit having a circuit for converting the air flow rate signal to a digital signal. The feature of the prior art resides in that a circuit for correcting the air flow rate signal according to a reference voltage of an A/D conversion circuit is inserted between the A/D conversion circuit and the air flow detection circuit. Thereby, it is possible to accurately perform the analog-to-digital conversion of the air flow rate signal, since the air flow rate signal can be corrected according to the variation of the reference voltage of the A/D conversion circuit.
However, the method in which a circuit for correcting the air flow rate signal according to a reference voltage of an A/D conversion circuit is inserted between the A/D conversion circuit and the air flow detection circuit, is not a way in which a highly accurate A/D conversion can be obtained, because there is an error in the correction circuit itself.
Further, while a highly accurate constant voltage source is used as a reference voltage of the A/D conversion circuit, the constant voltage source slightly fluctuates. Further, when the drive circuit and the detection circuit for detecting an air flow rate are over the operating voltage, they do not depend upon the voltage of the voltage source. Therefore, the value of the air flow rate converted from an analog signal to a digital signal has an error due to the fluctuation of the reference voltage of the A/D conversion circuit. For example, assumed that the reference voltage of the A/D conversion circuit is Vcc, the number of bits in the A/D conversion circuit is 8 and the value of the digital conversion is Dafs, the conversion equation is as follows.
Dafs=Vafs/Vcc*256 
Namely, owing to the reference voltage Vcc of the A/D conversion circuit, the converted value Dafs has an error.
An object of the invention is to provide an thermal air flow meter in which it is possible to perform with high accuracy the A/D conversion of an air flow rate signal, irrespective of the fluctuation of a reference voltage of an A/D conversion circuit.
To attain the above object, the present invention provides a thermal type air flow meter comprising a means for measuring intake air flow rate by controlling the heating of a heating resistor, and a means for performing the analog-to-digital conversion of the air flow rate signal of the air flow rate measuring means, wherein a reference voltage is input to the A/D conversion circuit and converted from an analog signal to a digital signal therein, and wherein the air flow rate signal is corrected according to the value of the reference voltage.
According to the present invention, it is possible to performing the A/D conversion of an air flow rate signal with high accuracy, even when the voltage of the reference voltage source of the A/D converter in an electrical control unit is fluctuating.